This invention relates to apparatus and methods for collision detection and recovery for waterjet and abrasive-jet cutting systems.
Waterjet and abrasive-jet cutting systems are used for cutting a wide variety of materials. In a typical waterjet cutting system, a high-pressure fluid (e.g., water) flows through a cutting head having a cutting nozzle that directs a cutting jet onto a workpiece. The cutting nozzle may include a mixing tube for introducing an abrasive into the high-pressure cutting jet to form an abrasive cutting jet. The cutting nozzle may then be controllably moved across the workpiece to cut the workpiece into the desired shape. After the cutting jet (or abrasive cutting jet) passes through the workpiece, the energy of the cutting jet is dissipated and the fluid is collected in a catcher tank for disposal. Waterjet and abrasive jet cutting systems of this type are shown and described, for example, in U.S. Pat. No. 5,643,058 issued to Erichsen et al. and assigned to Flow International Corp. of Kent, Wash., which patent is incorporated herein by reference. The ""058 patent corresponds to Flow International""s Paser 3 abrasive cutting systems.
FIG. 1 is an isometric view of a waterjet cutting system 10 in accordance with the prior art. The waterjet cutting system 10 includes a cutting head 20 coupled to a mount assembly 30. The mount assembly 30 is controllably driven by a control gantry 40 having a drive assembly 42 that controllably positions the cutting head 20 throughout an x-y plane that is substantially parallel to a surface 14 of a workpiece 12. Typically, the drive assembly 42 may include a pair of ball-screw drives oriented along the x and y axes and a pair of electric drive motors. Alternately, the drive assembly 42 may include a five axis motion system. Two-axis and five-axis control gantries are commercially-available as the WMC (Waterjet Machining Center) and the A and AF Series Waterjet cutting systems from Flow International of Kent, Wash.
FIG. 2 is a partial-elevational side view of the cutting head 20 and the mount assembly 30 of the waterjet cutting system 100 of FIG. 1. The cutting head 20 includes a high-pressure fluid inlet 22 coupled to a high-pressure fluid source 50, such as a high-pressure or ultra-high pressure pump, by a high-pressure line 23. In this embodiment, the cutting head 20 includes a nozzle body 24 and a mixing tube 26 terminating in a jet exit port 28. Although the term xe2x80x9cmixing tubexe2x80x9d is commonly used to refer to that portion of the cutting head of an abrasive jet cutting system in which abrasive is mixed with a high-pressure fluid jet to form an abrasive cutting jet, in the following discussion, xe2x80x9cmixing tubexe2x80x9d is used to refer to that portion of the cutting head 20 that is closest to the workpiece 12, regardless of whether the waterjet cutting system uses an abrasive or non-abrasive cutting jet.
The mount assembly 30 includes a mounting arm 32 having a mounting aperture 34 disposed therethrough. The mounting arm 32 is coupled to a lower portion 44 of the control gantry 40. The nozzle body 24 of the cutting head 20 is secured within the mounting aperture 34 of the mounting arm 32.
In operation, high-pressure fluid from the high-pressure fluid source 50 enters the high-pressure fluid inlet 22, travels through the nozzle body 24 and mixing tube 26, and exits from the jet exit port 28 toward the workpiece 12 as a cutting jet 16. The cutting jet 16 pierces the workpiece 12 and performs the desired cutting. Using the control gantry 40, the cutting head 20 is traversed across the workpiece 12 in the desired direction or pattern.
To maximize the efficiency and quality of the cut, a standoff distance d (FIG. 2) between the jet exit port 28 of the mixing tube 26 and the surface 14 of the workpiece 12 must be carefully controlled. If the standoff distance d is too close, the mixing tube 26 can plug during piercing, causing system shutdown and possibly a damaged workpiece 12. If the distance is too far, the quality and accuracy of the cut suffers.
The mixing tube at 26 is typically fabricated of specially formulated wear-resistant carbides to reduce wear. Particularly for abrasive cutting systems, the mixing tube 26 suffers extreme wear due to its constant contact with high velocity abrasives. Thus, mixing tubes are a relatively expensive component of the cutting head 20. The specially formulated carbides are also quite brittle, and can easily break if the mixing tube 26 collides with an obstruction during operation of the cutting system 10, such as fixturing or cut-out portions of the workpiece 12 which have been kicked up during the cutting operation. Accidental breakage of the mixing tube 26 increases operational costs and downtime of the cutting system 10.
Current collision sensors use a ring sensor disposed about the mixing tube 26 which slides along or slightly above the surface 14 of the workpiece 12. The ring sensor indicates the relative height of the workpiece. A motorized ball-screw drives the cutting head up and down to maintain the required standoff distance. When the ring collides with a kicked-up part or other obstruction, a detector detects the collision and sends a stop signal to the control gantry to stop the movement of the mixing tube in an attempt to avoid the collision.
A fundamental problem with such collision sensors is that they must have a large enough xe2x80x9csafety bufferxe2x80x9d between the sensor and a mixing tube to allow the control gantry enough time to stop without damaging the mixing tube. Due to the size and speed of modern cutting systems, the task of stopping the control gantry quickly to avoid a collision is quite difficult. Another problem is that any shifting of the components requires a lengthy re-calibration routine to ensure the proper standoff distance d. A serious collision can irreparably damage the ring sensor.
One approach has been to simply make the ring larger the allow to control gantry more time and room to stop. This approach, however, prevents the cutting jet 16 from cutting near obstructions and fixtures commonly found around the edges of the workpiece 12, thereby wasting material. Enlarging the ring also increases the occurrence of erroneous collision signals which results in unnecessary downtime of the cutting system. Finally, existing ring sensor devices are expensive and are not robust in detecting surface height or collisions when operating the control gantry at high-speed or under dirty conditions.
This invention relates to apparatus and methods for z-axis control and collision detection and recovery for waterjet and abrasive-jet cutting systems. In one aspect of the invention, an apparatus includes a linear rail, a slide member coupleable to the cutting head and slideably coupled to the linear rail, at least one actuator having a first end coupled to the slide member and a second end fixed with respect to the linear rail, a position sensor coupled to the slide member, and a controller. The actuator provides an adjustable support force that supports the weight of the cutting head, allowing the cutting head to be controllably positioned at a desired height above the workpiece. The actuator may include a pneumatic cylinder, or alternately, a linear motor.
In another aspect, an apparatus according to the invention includes a first mount member coupleable to a controllably positionable mounting surface of the waterjet cutting system, a second mount member coupleable to the cutting head and disengageably coupled to the first mount member, and a sensing circuit having a plurality of first conductive elements disposed on the first mount member and a plurality of second conductive elements disposed on the second mount member. In the event of a collision between the cutting head and an obstruction, the second mount member disengages from the first mount member to prevent breakage of the cutting head. Following the collision, the second mount member is quickly and easily re-engaged with the first mount member without time-consuming re-calibration. In one embodiment, re-engagement of the second and first mount members is automatically performed by a biasing member.
In another aspect, an apparatus according to the invention includes a first mount member coupleable to a controllably positionable portion of the waterjet cutting system, and a second mount member coupleable to the cutting head. The second mount member is rotatably engaged with the first mount member. In the event of a collision between the cutting head and an obstruction, the second mount member rotates with respect to the first mount member and the waterjet cutting system to prevent breakage of the cutting head. A biasing member coupled to the second mount member urges the second member back to the proper orientation for operation. The system can incorporate an open sensing circuit having a first contact coupled to the second mount member and a second contact coupled to ground. The first contact is adjustably spaced from the second contact such that rotation of the second mount member caused by displacement of the cutting head during collision results in the first contact touching the second contact, thereby closing the sensing circuit. Accordingly, a collision by the cutting head results in a signal that, for example, stops movement of the gantry to prevent damage to the cutting head.
In another aspect, a method of controlling a height of a cutting head of a waterjet cutting system over a surface of a workpiece includes coupling a first end of a contact member to the cutting head, engaging a second end of the contact member with the surface of the workpiece, providing an adjustably controllable support force to support a weight of the cutting head, and slightly reducing the support force to slightly downwardly bias the contact member into engagement with the surface of the workpiece. The position control method advantageously provides a simple height measurement system and also allows for automatic adjustment for changes in friction or weight of various components of the waterjet cutting system.